Intermittent Fasting: Does It Affect Sports Performance? A Systematic Review

1. Introduction

Different recent reviews and high-level sports nutrition bodies highlight the role that nutrition plays today in the sports performance of athletes in each of the different sports disciplines [1,2].

However, in recent years different dietary patterns and protocols have emerged that have tried to amplify or reduce the adaptations derived from physical exercise to try to achieve an improvement in the athlete's sports performance [3].

Within these strategies, intermittent fasting has been acquiring special renewed interest due to its supposed effects on health and improvement of body composition in patients with different pathologies of great predominance nowadays [4,5]. This dietary protocol focuses on timed fasting/eating periods with different time intervals [4,6].

Despite this recent popularity, currently its effects on performance do not seem to be clarified, having special importance in the physical and cognitive performance of those athletes who frequently perform this type of dietary patterns or temporary caloric restrictions as it happens with Islamic athletes during the religious practice of Ramadan [6,7,8].

Therefore, the main objective of the present review will be to investigate whether intermittent fasting intervenes in performance in professional athletes, evaluating the effects of this dietary protocol on aerobic and anaerobic capacity, strength and power and body composition (fat mass, muscle mass, weight).

2. Materials and Methods

This work is a systematized literature review based on scientific evidence documenting the effects of intermittent fasting in athletes and/or people who perform physical activity and its benefits on sports performance. This review was registered in PROSPERO with code ref. 407024, and an evaluation of the quality or risk of bias was performed by MM-B. For the literature review the following databases were used; MEDLINE through PubMed search engine, Web Of Science and Scopus. The keywords used were: "intermittent fasting" or "intermittent fasting", "sport" or "sport", "performance" or "performance" and "exercise" or "exercise" where 4,402 articles were obtained using as Boolean connectors "or" between the words in different languages and "and" between the different ones (“intermittent fasting” OR “intermittent fasting” AND “sport” OR “sport” AND “performance” OR “performance” AND “exercise” OR “exercise”). The exclusion criteria that have been established are the following: dates established between 2013 and the present, free full texts, studies conducted in adult human athletes, languages of the articles in English and/or Spanish.

As for the inclusion criteria of the articles, it has been considered that intermittent fasting is mainly linked to sports practice and that this obtains a result in terms of performance or physical capacities.

Therefore, when filtering articles from 2013 to today, being in humans and adults and limiting the languages to Spanish and English 114 articles were found. After eliminating duplicates, we obtained 92 papers, once the title was read, 34 were selected and 58 were eliminated because the title was not related to the topic of the systematic review. After reading the abstract and observing the bibliography, 12 articles were eliminated, but 3 were added from the bibliography obtained for a complete reading. This whole procedure is followed according to PRISMA standards [9] by means of which a flow chart is made (Figure 1).

3. Results

Within the bibliography of this review, we found that the majority highlighted that the subjects were athletes [10,11,17,21,22,23,24,25,31,32,33,34]. Another did not specify if they were athletes [30], but due to their intervention it was included, while the rest indicated that the subjects were physically active [14,18,26,27,28].

Most of them showed time-restricted feeding (TRF) with a 16-hour fasting window and 8-hour feeding window [10,11,14,16,17,18,21,23,25,26,33]. Other studies analyzed 14/8 fasting during the Ramadan period [12,21,32], while another article analyzed overnight fasting [24].

In addition to this, it should be noted that two studies used more variables in their studies, such as the intake of supplementation in addition to the fasting follow-up, namely hydroxy methyl butyrate (HMB) and two types of whey protein, whey protein concentrate (WPC) and hydrolyzed whey protein (WPH) [26,29]. Another article sought to compare the difference between protein-loaded and carbohydrate-loaded fasting [34]. For the most part, a control group with no fasting and an experimental group performing fasting are identified [18].

Based on performance, specifically aerobic performance, 8 articles were included that used different tests (Table 1), among them: 20 minutes cycling test [29], 10 km test [17], repeated sprints test [24,28], treadmill test [25], test at 45% of maximum power [10].

Regarding anaerobic performance, 6 articles (Table 2) used different tests to evaluate different parameters of the sample such as stress tests [10,17], Wingate test [14,28,29], submaximal exercise [34], repeated sprints test [28] and interval training [34].

Table 3 indexes the 8 included studies that evaluated muscular strength through maximal strength and endurance strength tests, and power through peak power (PPO), average power (W) [11,16,23,26,27,28,32,33].

Finally, Table 4 included the 15 studies that evaluated any body composition variable such as body fat mass, lean mass, and anthropometric folds [10,11,12,13,15,17,18,19,20,23,25,26,27,30,31,32,33].

4. Discussion

To begin with, we must consider that there is a wide variety of studies chosen in the review in terms of the difference of sports in the sample, the differences in gender and age and the different interventions.

Firstly, we can affirm that intermittent fasting (any type) affects the improvement of body composition [10,11,16,17,18,21,23,26,27,28]. With this, we can concretize that it intervenes positively in performance, since a reduction in body weight would be considered beneficial [10,17,27].

Intermittent fasting could be considered as an adequate nutritional strategy to reduce fat mass and body fat percentage to an adequate number for the athlete (between 6 - 12% in men and between 12 - 18% in women) and maintaining lean mass or muscle mass [1,2,8,12]. Martinez-Rodriguez et al. (2021) [27] in their study with 14 active women who combined TRF with high intensity training (HIIT) compared to HIIT with normal diet, observed a significant decrease in fat in the fasting group. It should be noted that the food log showed a non-voluntary caloric restriction of 10-20% per week. Hosseini et al. (2015); in their study with 50 healthy subjects combining a fasting intervention together with the performance of physical activity [12] observed a decrease in body mass index and body fat percentage.

It should be clarified that the effectiveness of the fasting protocols included in this review is closely linked to the time of application and to the type of population. Fasting is more effective in the medium and long term than in the short term [14,27].

In turn, we should consider that in some studies [18,23,25,26,27] caloric restriction is combined with the follow-up of intermittent fasting. This aspect highlights conclusions obtained from recent studies where no superiority of different dietary protocols is observed, including intermittent fasting, if an adequate caloric deficit and a correct adherence to the dietary plan is reached [35,36]. Therefore, the effect of caloric restriction is a very important element to consider in relation to the reduction of fat mass.

Despite this, studies such as the of Isenmann et al. study show a greater adherence to the diet in those subjects who followed a nutritional plan with intermittent fasting compared to a diet without restrictions [18]. This aspect could be interesting onsidering the important role of a higher adherence for the adequacy of a long-term fat loss and the need for individualization of the same [37].

Considering health in conjunction with intermittent fasting, Perez-Montilla et al. (2022) in their review points out that fasting controls body weight, improves insulin sensitivity, reduces systemic inflation and strengthens the immune system [21]. Thus, it can be linked to injury prevention and recovery. Several reviews mention benefits of IF in improving metabolic health and insulin sensitivity [13], as well as in regulating glucose and certain lipid metabolism [19]. Certain metabolic processes in adipose tissue are mediated by endogenous clocks in our body such as adiponectin, the levels of this hormone can be altered by changes in sleep/wake or feeding/fasting cycles [19]. The TRF type of fasting appears to be an effective strategy to improve the levels of this hormone. Moro et al. (2016) in their randomized clinical trial, 34 men habituated to resistance training [11] found an increase in adiponectin levels and a decrease in leptin levels. The same author in another study with 16 cyclists [10] observed a trend of higher increase in adiponectin levels in the TRF group (+33%) compared to (+8%) in the normal diet group. Low levels of adiponectin have been associated with obesity, oxidative stress [11] and insulin resistance, whereas higher levels are associated with improved insulin sensitivity in adipose tissue [19].

This improvement in the treatment of different diseases has also been contrasted by other trials [38,39]. However, recent reviews show the need for long-term studies evaluating this type of intervention versus isocaloric restriction in humans to know whether the results may be different from those observed after similar weight loss achieved through modest continuous energy restriction [40,41,42].

Respect to the levels of certain inflammatory markers some studies hypothesize that fasting may activate cellular mechanisms that enhance immune function [4]. In a group of elite cyclists, interleukin (IL-6) appeared to decrease in the TRF group with an upward trend in the normal diet group [10]. Also, in this same study he observed a decrease in the neutrophil-lymphocyte ratio (NLR) in both groups, but it was significant relative to baseline values only in the TRF group. NLR is a biomarker of systemic inflammation that correlates with blood C-reactive protein levels, which were found to be decreased during Ramadan fasting in two studies of soccer players [31].

Finally, in terms of performance, we found both positive and negative aspects. It seems that during Ramadan (14/10), negative effects appear in terms of performance, which may be due to other aspects such as rest and hydration. Greater negative effects can be observed in elite athletes, but in amateur athletes there is not much difference [21,32,33]. On the other hand, in the TRF modality (16/8), there do not seem to be differences in the performance of physical capacities: aerobic [10,16,17,22,25,29], anaerobic [17,22,28,29,33], strength and power [11,14,16,21,23,26,29,33,34]. The results of this review thus show that, in terms of capacities, fasting does not have a negative impact on sports performance and could be considered an adequate type of diet for sports practice.

Recent reviews have also highlighted these varied findings, stressing the need for more research in this regard given the wide heterogeneity of protocols and measurable variables [33,43].

Nevertheless, the possible effect of this type of interventions on the athlete's body composition and therefore, its possible link with the athlete's performance should be considered [16,29].

Finally, the possible limitations observed during the elaboration of the work are presented, and due to these, it is not possible to obtain accurate conclusions.

The review has had little variety of interventions, which does not allow us to identify a specific action protocol. The samples, tests, measurements, objectives, and variables of the different studies have also been a limitation. Some of them did not count calories during the interventions and because of this we cannot obtain a reliable conclusion about the results obtained, or in the case of the variables, not all the studies measured the same ones, so we cannot draw reliable conclusions in their entirety. Many of the articles look for short term effects, when it has been concluded that long- and medium-term effects are more effective. Most of the studies conclude that a follow-up of future interventions is necessary to continue monitoring the effects of fasting, so there seems to be a lack of evidence in the studies.

5. Conclusions

After carrying out this systematic review of the studies and literature on how intermittent fasting affects athletes and sports performance, the questions and objectives posed at the beginning of the study can be answered. Referring to the main objective, the scientific evidence indicates that intermittent fasting does not negatively affect sports performance and does affect the improvement of body composition. Therefore, it may be an appropriate strategy for sports or athletes seeking a control of lean mass and a decrease in fat mass, which can have a positive transfer in terms of performance, since by reducing their weight and specifically their fat mass they will be able to perform better in certain situations in which power capacity is used.

The performance of the aerobic capacity would be benefited since the improvement of the corporal composition can help to the improvement of the resistance; this fact ends in an improvement of the power being related directly with the corporal weight. As far as strength is concerned, it is not compromised by intermittent fasting and remains at the same level. Intermittent fasting would be related to an increase or improvement of health and the immune system; this is also related to performance as it improves the ability to prevent injuries and their recovery. Further studies would be needed to be more certain about the effects of intermittent fasting on sports performance. The heterogeneous samples make it difficult to have a definitive conclusion, but the data are favorable, and it could be recommended to follow intermittent fasting without reducing performance in athletes.

In the case of Ramadan is different, it can be considered as a type of intermittent fasting/TRF in which the fasting period varies between 12 and 18 hours depending on the season and location, unlike the other types of TRF is not allowed the intake of any type of liquid and the fasting time lasts from sunrise to sunset. This causes disturbance in the sleep-wake and rest-activity cycles. Due to changes in sleeping and eating hours physiological and psychological disturbances can be caused. There is evidence showing that fasting during Ramadan affects a decrease in performance in the Yo-Yo test that measures VO2 max [14].